The last few years have seen astonishing progress in the field of optical fiber communications. For instance, low loss (e.g., about 0.3 db/km) silica-based optical fiber, the transmission medium of present choice, is now routinely produced. In fact, silica-based single mode fiber whose loss is only a few hundredths db/km above the theoretical minimum has been achieved.
Fiber loss is a parameter of great economic significance since it determines, for instance, the maximum distance between amplifiers or repeaters in a given transmission path. Thus it is highly desirable to have available fiber of the lowest possible loss. It is well known that optical fiber comprises a core of relatively high refractive index that is contactingly surrounded by a cladding having relatively low refractive index.
It is known that some non-SiO.sub.2 -based glasses (e.g., fluoride and high-alkali alumino-silicate glasses) could have lower loss than SiO.sub.2 if they could be manufactured in sufficient purity. To date, however, this has not been possible. Furthermore, such fibers likely would demand treatment and procedures that differ radially from those now established in the industry.
It is generally accepted in the art that the addition of a small amount of alkali metal oxide to vitreous SiO.sub.2 raises the total intrinsic attenuation of the resulting glass above that of pure SiO.sub.2 (which is now known to be about 0.15 db/km at.lambda.=1.55.mu.m). See, for instance Y. Schroeder et al., Journal of the American Ceramic Society, Vol 56(10), pp. 510-514, especially FIG. 2, which shows a large peak in the Landau-Placzek ratio for low concentrations of K.sub.2 O in SiO.sub.2. See also "Treatise on Materials Science and Technology", Vol. 12, M. Tomozawa et al, editors, Academic Press 1977, especially pages 183 and 184, which show similar data for K-doped (FIG. 5) and Na-doped (FIG.6) SiO.sub.2.
U.S. Pat. No. 4,666,247 (incorporated herein by reference) discloses SiO.sub.2 -based optical fiber comprising a "modifier" chosen from Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, and the rare earths, and further comprising a "homogenizer" chosen from B, Al, Ga, In, P, As and Sb. The patent teaches that the presence of the homogenizer permits achievement of relatively high modifier concentrations that lead to phase separation or crystallization in prior art fibers. S.R. Nagel et al. (Journal of the American Ceramic Society, Vol. 59(1-2), p 47, 1976) describe a technique for making fiber having a potassium silicate core and a silicate cladding, with K serving as the index-raising dopant. The fibers had minimum loss (10 db/km) at about 0.65 .mu.m and had relatively large core size (up to 40 .mu.m diameter). Consequently, the fibers were not single mode fibers at any of the wavelengths disclosed in the paper.
For the above discussed and other reasons it would be highly desirable to have available optical fiber that has a lower intrinsic loss than prior art SiO.sub.2 -based fiber at the important communications wavelengths of about 1.31 and for 1.55 .mu.m, but that in all other respects acts substantially the same as the prior art fiber. This application discloses a fiber that can meet these requirements. Typically the fiber is a single mode fiber at the desired operating wavelength.